Page 3 - Achievement of Field-Reversed Configuration Plasma Sustainment via 10 MW Neutral-Beam Injection on the C-2U Device

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scale strongly with NB input power; and (v) plasma performance correlates with NB pulse duration in which diamagnetism persists several milliseconds after NB termination due to accumulated fast ions. The key accomplishment on C-2U is sustainment of advanced beam- driven FRCs with a macroscopically stable and hot plasma state for up to 5+ ms, limited only by hardware and stored energy constraints such as the NB’s pulse duration and current sourcing capability of end-on plasma guns. In this well-sustained FRC regime fast ions are almost classically confined and then suppressing broadband magnetic turbulence as well as density fluctuations near the separatrix and in the scrape-off layer (SOL) by a combination of NBI and E×B shearing via plasma-gun edge biasing, thereby improving confinement properties [9]. There appears to be a strong positive correlation between Te and energy confinement time, and particle confinement time is more than 10× greater than that of conventional FRC scaling predictions [10].
In this paper, C-2U experimental apparatus and diagnostic suite are described in Section 2. Key systems / elements to obtain HPF operating condition as well as detailed characteristics of newly-obtained advanced beam-driven FRCs are described in Section 3; in addition, key C-2U experimental results including plasma sustainment are also discussed. Lastly, a summary is provided in Section 4.
2. C-2U Experimental Device and Diagnostic Suite
The C-2U device, shown in Fig. 1(a), is a large theta-pinch, CT-merging system, built at Tri Alpha Energy to form relatively high flux, high temperature FRC plasmas [4,8]. Figure 1(b) illustrates typical FRC magnetic flux and density contours in the C-2U device; these contours are obtained from a two-dimensional magnetohydrodynamic (MHD) numerical simulation performed with the LamyRidge equilibrium code. The C-2U device has ~20 m in overall length and consists of a central confinement region surrounded by two field-reversed theta- pinch (FRTP) formation sources and two divertors. The stainless-steel confinement chamber (inner-wall radius, rw ~0.7 m) approximately conserves magnetic flux inside the vessel wall; however, for long-lived plasma discharges magnetic-flux leakage needs to be taken into account for accurate magnetic-field calculations and other associated and post- processed plasma/physics parameter calculations. The formation tubes are made of quartz, which are approximately 3.5-m long and 0.6-m in diameter; the C-2U vacuum vessel accommodates ultrahigh vacuum. A set of DC magnets generates a quasi-static axial magnetic field, Bz, throughout the device, for which we can arrange a coil/power-supply configuration to control the axial-field profile and amplitude. The typical magnetic field is Bz ~0.1 T in the confinement region with an end-mirror ratio of 3.0–3.5. There are magnetic mirror plugs in between the formation and divertor sections at each side that can produce a strong magnetic field up to ~1.5 T, which corresponds to a plug-mirror ratio of ~15 compared to the central confinement section. The mirror plugs play an important role in contributing to the open-field-line plasma confinement as well as assisting plasma-gun operation inside of the end divertors. As shown in Fig. 1(b), two coaxial plasma guns are located on axis inside of each divertor, and there are concentric annular electrodes behind the plasma guns to control open-field-line connection/contact to divertor vessel wall. Both, plasma guns and electrodes, are important for edge biasing as well as radial electric field control in C-2U. Six brand new C-2U NB injectors, located in the confinement vessel, were substituted for those used in C-2, comprising the following key changes: increased total NB input power from ~4 MW (20 keV hydrogen) to 10+ MW (15 keV hydrogen) with higher current at lower beam energy; tilted NB injection angle in a range of 65°–75° relative to the machine axis to improve coupling between the beams and the target FRC plasma.